245/50R18 tyre size
A 245/50R18 tyre is 245 mm (9.6") wide with a sidewall height of 50 % of the width, i.e. 122.5 mm (4.8"), on a 18" (457 mm) rim. Overall diameter is 702.2 mm (27.6"), so the closest inch-style size is 27.6×9.6R18. Static circumference is 2206 mm; the ETRTO rolling circumference is 2142 mm, giving 467 revolutions per km (751 per mile).
Dimensions and rolling diameter
| Section width | 245 mm | 9.65″ |
| Sidewall height | 122.5 mm | 4.82″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 702.2 mm | 27.65″ |
| Static radius | 351.1 mm | 13.82″ |
| Static circumference (π × D) | 2,206 mm | 86.9″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,142 mm | 84.3″ |
| Effective rolling radius | 340.9 mm | 13.42″ |
| Revs per km (rolling / static) | 467 / 453 | |
| Inch equivalent | 27.6×9.6R18 | |
| Approved rim widths (approx.) | 6.5″–8.5″ (measuring rim 7.5″) | |
A 245/50R18 tyre is 245 mm (9.6") wide with a sidewall height of 50 % of the width, i.e. 122.5 mm (4.8"), on a 18" (457 mm) rim. Overall diameter is 702.2 mm (27.6"), so the closest inch-style size is 27.6×9.6R18. Static circumference is 2206 mm; the ETRTO rolling circumference is 2142 mm, giving 467 revolutions per km (751 per mile).
Why two circumferences? The static figure is simple geometry (π × diameter). Under load a radial tyre flattens and rolls about 3 % less per revolution; ETRTO defines the rolling circumference as 3.05 × the nominal diameter, which is what manufacturers quote and what speedo and gearing calculations should use.
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